I realize that Manning has had it pretty rough while in custody, but: 1. Snowden is a civilian. He has not submitted himself to military law. As far as I am aware, everything that has happened to Manning is (unfortunately) in line with military law. 2. Manning leaked GBs of confidential documents seemingly at random. Snowden is pointing out a particular problem which he finds morally repulsive. These are two different issues.
In short, the more Snowden acts like a criminal, the more he will be perceived to be a criminal. Monitoring citizens is one thing. The failure of the criminal justice system / law and order is another. Furthermore, it is difficult to support him when he is in hiding.
Snowden is acting like he did something wrong. He should own it.